In today's dimmable fluorescent lighting market, a number of different methods are used for dimming control. One popular method for dimming control employs a dimmer control interposed between a power line and an input of a dimming ballast. The dimming control comprises a phase-control device, such as a triac, to modify a firing phase angle of an alternating current (AC) powering signal. A dimming ballast circuit, in turn, controllably dims a fluorescent lamp based on the firing phase angle.
In some applications, the aforementioned dimming control approach yields an undesirably-high total harmonic distortion (THD) and an undesirably-low power factor. The high THD is caused by the chopping action of the triac. As a result, applications of the aforementioned dimming control approach have been limited.
U.S. Pat. No. 5,872,429 discloses use of coded perturbations in the line signal to obtain a lower THD. An encoder encodes a command over a command period of several cycles in the line signal. The encoder encodes the command by selectively injecting perturbations near zero-crossings of specific cycles in the command period. A controller within a ballast detects the perturbations over the command period, and decodes the command. The perturbations may be injected only when a change of light level is needed.